The present invention relates to a phosphate glass for use as an infra red (IR) filter and to a method of forming the same, and is particularly but not exclusively directed towards filters for use on external lights for aircraft, ships and the like.
Aircraft lights, whether fitted to civil or military aircraft, generally comprise navigation lights at the wingtips (red port and green starboard) and white tail lights. High intensity anti-collision strobe lights are also fitted at the top and bottom of the fuselage.
Strict regulations operate to govern the color and intensity of aircraft lights. In the United Kingdom, the Civil Aviation Authority ("CAA") has responsibility for ensuring that all aircraft adhere to the regulations. For example, at least in relation to civil aircraft, the red and green wingtip lights must fall within stipulated color bandwidths, so that an aircraft whose lights emit an orange or a blue hue, rather than red or green, would fail to meet CAA regulations and hence be refused a licence.
Although military aircraft are exempt from these regulations, it is obviously desirable that lights on military aircraft conform as closely as possible to the specified standards.
To improve visibility under low light conditions, it is now common for pilots to fly wearing night vision goggles. These goggles are fitted with a filter to exclude light in the visible spectrum and operate by detecting radiation in the infra red ("IR") region (650 nm to 1000 nm). To maintain a good output image under varying light levels, the goggles are generally provided with automatic gain control.
While the automatic gain control operates satisfactorily in most conditions, it is unable to compensate when a very bright light is introduced into the field of view. Such a situation occurs, for example, when the pilot approaches another aircraft at night and results in the output image becoming very bright and occasionally the whole display is "bleached".
The inability of the goggles to cope with bright lights at night can therefore make it difficult for the pilot accurately to locate other aircraft in the vicinity and, at worst, may temporarily blind the pilot. In such circumstances, it will be appreciated that both aircraft can be placed in an extremely dangerous situation.
On the other hand, if other aircraft are to remain clearly visible and identifiable as such when viewed through night flying goggles, it is preferred that at least some IR radiation is permitted to emerge from the lights. In other words, the level of IR emission should ideally be suppressed but not entirely eliminated.
It will be appreciated that there is a fine balance to be struck between filtering sufficient IR radiation to avoid temporarily blinding the pilot yet transmitting enough IR radiation to allow the pilot to recognize lights on other aircraft in the vicinity.
To the best of our knowledge, no-one has been able to quantify the intensity of the IR transmission which meets the above criteria. Experts in the field do, however, recognize when an appropriate balance has been achieved. In this regard, the balance is considered to be about right when the effect around each aircraft light as viewed through the goggles resembles a "football". This phenomenon is the commonly used indicator by which the optimum IR transmission level is judged.
Of course, while the aforementioned problems are most acute for pilots flying close to other aircraft, it is also important that pilots are able clearly to view other structures in low visibility conditions, for example when approaching naval vessels such as aircraft carriers, marine structures such as off-shore oil and gas rigs, or land-based constructions such as airports or even high-rise buildings.
A further factor which must be taken into account when filtering IR radiation is the potential for the filter medium to affect the light emitted in the visible spectrum. In this regard, it can happen that attenuation of IR radiation is accompanied by a color shift in the visible range. With regard to wingtip lights, the amount of color shift can mean the difference between compliance with or failure to meet CAA regulations.
Attempts to solve the aforementioned problems of filtering out a proportion of the IR radiation yet avoiding significant color shifts have only been partially successful as outlined below.
For example, lenses for aircraft lights have hitherto been made only from conventional silicate glass because this has been the only glass suitable for forming into curved sections which are then bonded together to form the lenses. In this regard, silicate lenses are generally formed by molding or pressing while the glass is in the molten state.
Although IR filter glass of the silicate type is commercially available, it is of limited use for aircraft lights because the light emitted in the visible spectrum is also affected. In this regard, it produces a measurable color shift in the red region making it difficult to comply with the CAA regulations.
Rather than using the aforementioned IR filter glass, an alternative has been to provide a coating of an IR filter material on conventional silicate glass. The high temperatures involved in forming the glass sections for subsequent assembly into the lens means that such coatings can only be effectively applied after the forming stage. Moreover, to avoid accidental damage to the coating, it is preferable to apply the coating after the individual sections have been bonded together to form the complete lens.
However, coating with an IR filter material after assembly of the lens has still proved unreliable because of the highly contoured surfaces involved. In particular, it has been difficult to coat either the interior or exterior surfaces of the lenses uniformly.
In order to be effective, IR filter coatings must be evenly applied; too thin a coating will result in inadequate IR attenuation with potentially disastrous consequences and too thick a coating may block out the IR spectrum completely. Coating of lenses made from conventional silicate glass is therefore problematic.
As far as other types of glass are concerned, it is known to dope phosphate glass with copper in order to achieve low transmission in the IR range and such a glass has been used to shield illuminated color displays such as those in aircraft cockpits. IR filter glass of this type, for example as is documented in U.S. Pat. No. 5,173,212 commonly assigned to Schott Glaswerke, is commercially available. However, for external lighting applications the glass would need to be only 1 mm thick, rendering it useless mechanically; in particular, because of its inability to withstand the treatment involved in known glass toughening processes. Such treatment is essential if it is to achieve the strength and durability required for the demanding physical conditions encountered when used on aircraft exteriors.
Moreover, in its known applications phosphate glass is characterized by its brittle nature and hence it has not hitherto been possible to form into substantially non-planar components.